Antarctic ice melt can change global ocean circulation, sediment cores suggest - Earth - Earth Sciences - Environment

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A new study shows that during the last two deglaciations, i.e., the transition from an ice age to the warm interglacial periods, meltwater from the Antarctic ice sheet intensified stratification in the Southern Ocean. The results highlight the key role of the Antarctic ice sheet on ocean circulation and the regulation of the global climate. The study was led by François Fripiat, a researcher at the Max Planck Institute for Chemistry and the Université Libre de Bruxelles, and was conducted in collaboration with researchers from Princeton University and the Alfred Wegener Institute. It is published in the Proceedings of the National Academy of Sciences.

Over the past 3 million years, Earth's climate has alternated between long glacial periods—during which immense polar ice sheets covered much of the Northern Hemisphere, extending as far as the European continent—and warmer interglacial periods. The transitions between these two states, known as deglaciations, were marked by gradual ice sheet disappearance.

"While the impact of melting large Northern Hemisphere ice sheets on North Atlantic circulation has been studied for decades and is recognized for its major climatic consequences, the specific role of Antarctica in the Southern Ocean that surrounds it remains largely unknown," explains Fripiat.

The Southern Ocean occupies a central place in the global climate system. It represents a true crossroad of ocean circulation, it connects the Atlantic, Indian, and Pacific Oceans. It is also the main exchange zone between the atmosphere and the deep ocean—a vast reservoir that stores about one hundred times more carbon dioxide than the atmosphere.

These exchanges depend largely on ocean stratification, that is, the way water masses are organized into layers that are more or less well mixed. "The ocean can be compared to a huge machine that redistributes heat and carbon on a planetary scale. When this machine becomes stratified, its operation slows down, with direct consequences for climate," explains Fripiat.

Diatoms as a climate archive

For the study, the researchers analyzed sediment cores taken from the Southern Ocean. Their data are based on the isotopic composition of organic matter preserved in the shells of diatoms. These microscopic marine algae are found in large numbers in the Southern Ocean sediments and serve as a natural archive of past environmental conditions.

Impact of ice-sheet melt on ocean mixing

The results show that during deglacial periods, ocean stratification intensified strongly near Antarctica, driven by large inputs of freshwater from ice-sheet melting. At the same time, farther north near the polar front, the combined action of these freshwater inputs and the westerly winds promoted enhanced upwelling of deep waters, maintaining a certain degree of ocean ventilation on a global scale.

"Our data show that the climate system did not completely grind to a halt. Even when the ocean near Antarctica became more stratified, other mechanisms still allowed deep waters to rise and exchange with the atmosphere, notably under the influence of winds. These exchanges may have released CO₂ into the atmosphere, contributing to the warming that ended ice ages," explains Fripiat.

Far from being a simple icy desert, Antarctica thus appears as one of the invisible conductors of Earth's climate system. Understanding its mechanisms means better anticipating the planet's future. 

Provided by Max Planck Society 

by Claudia Dolle, Max Planck Society

edited by Lisa Lock, reviewed by Robert Egan 

Source: Antarctic ice melt can change global ocean circulation, sediment cores suggest